It has been found useful in agriculture, construction, road building, and other related fields of civil engineering to create a reference plane of light, usually coherent light from a laser source, in a region where work is to be done. The plane is received by an electro-optical receiver to determine the height of the blade of a bulldozer or ditching machine, for example, to enable precise construction work to be done by relatively unskilled operators.
Prior art receivers for these applications have consisted of rows of photocells directed outwardly at different levels around the surface of a cylinder having a vertical axis, or rows of fiber-optics bundles similarly arranged. These arrangements are provided to avoid the necessity of re-orienting the receiver in azimuth as the direction of movement of a bulldozer changes, for example, and to enable determination of the direction of any vertical movement of the receiver.
Such arrangements are complex and require much assembly labor, and hence are expensive. They also are sensitive to ambient light in a wide vertical acceptance angle, as well as to the light of the referenced plane.